Brake systems of motor vehicles today are generally designed as hydraulic brake systems. In such systems, a brake master cylinder is provided, in which a pressure is generated in a pressure chamber filled with brake fluid by displacement of a piston connected to a brake pedal. The pressure is passed, by means of hydraulic lines, to at least one wheel brake cylinder. The wheel brake cylinder brings, for example, brake linings mounted on brake pads in contact with a brake drum or a brake disk as a result of the pressure of the brake fluid. This enables the braking force acting on the wheel connected to the brake drum or brake disk to be regulated by operating the brake master cylinder. The brake master cylinder is thereby normally operated by using the brake pedal, wherein a braking force booster can be connected to the piston rod of the brake master cylinder for boosting the braking force. The brake master cylinder is generally designed as a tandem brake master cylinder, which comprises two pressure chambers for applying pressure to two independent brake circuits. The two brake circuits can each act upon all wheels of the motor vehicle, but also can, for example, be divided between the individual wheels such that one of the brake circuits acts on the front wheels and the other on the rear wheels.
Because of the importance of the brake system for the safety of the motor vehicle, it is important to recognize possible fault conditions of the brake system, automatically if possible. One such fault condition is the presence of undissolved gas in the brake fluid, e.g., in the form of air bubbles. The presence of air bubbles in the brake fluid restricts the operability of the brake and the maximum achievable brake fluid pressure. That is, air in the brake system impairs the pedal feel and limits the maximum output pressure. Another fault condition is a rearward displacement of a piston of a wheel brake cylinder (also known as piston knock back). The pedal travel necessary for generating a braking effect is greatly increased by this, which is hardly controllable by the driver. That is, piston knock back increases the amount of pedal travel required to achieve deceleration, which is very hard for the driver to control. Finally, a third fault condition is a failure of a brake circuit. The achievable braking effect is likewise greatly reduced by this, which has to be taken into account as early as during the design of the brake circuit. In particular, loss of one circuit is the limiting factor for brake system sizing for a front rear split brake system. These three phenomena have a strong interaction which makes them difficult to distinguish from one another. For this reason, known brake systems do not provide sufficiently reliable detection of all the fault conditions described above.
With this in mind, the object of the present disclosure is to provide a method for the detection of a fault condition of a hydraulic brake system of a motor vehicle and a hydraulic brake system for a motor vehicle, wherein the disadvantages mentioned above are avoided.